A progressing cavity pump has a stator and a rotor. The stator typically comprises an elastomeric liner within a housing. The stator is open at both ends and has a double helical passage extending through it. The rotor is normally of metal and has a single helical exterior formed on it. Rotating the rotor causes fluid to pump through the stator. Progressing cavity pumps are used for a variety of purposes.
As a well pump, progressing cavity pumps may be driven by a downhole electrical motor or by a string of rods extending to a motor located at the surface. With a rod driven pump, normally the stator is suspended on a string of tubing, and the drive rods are located within the tubing. When installing a rod driven progressing cavity pump, the operator first secures the stator to the string of tubing and runs the tubing into the well to a desired depth. The operator then lowers the rotor through the tubing on the string of rods and into the stator.
To operate the pump at desired capacity, the rotor must be at the desired axial spacing within the stator and the rods must be in tension. If the lower end of the rotor is spaced above a lower end of the stator during operation, then a lower portion of the stator will not be in engagement with the rotor and the pumping capacity will suffer. The operator thus needs to know when the rotor has fully entered the stator during installation. The operator can calculate how much the rods will stretch due to the hydrostatic weight of the column of well fluid in the tubing. With the anticipated stretch distance known and with the rotor at a known initial position in the stator, the operator can pull the rods and rotor upward a distance slightly greater than the anticipated stretch, so that during operation, the rotor will move back downward to the desired axial position relative to the stator.
In the prior art, prior to running the tubing, the operator secures or welds a tag bar across the bottom of the stator. During installation, downward movement of the rods will stop when the lower end of the rotor contacts the tag bar at the bottom of the stator. Upon tagging the bar, the operator pulls the rod string back toward the surface by the calculated amount of rod stretch. During operation, as well fluid fills the tubing, the rod stretches, allowing the rotor to move back downward until in full engagement with the stator. If installed properly, once the rods have stretched fully, the lower end of the rotor will be spaced above the tag bar and the rods will be in tension.
While this method works well enough, tag bar creates an obstruction at the bottom of the pump. The obstruction prevents the operator from lowering tooling or instruments through and below the pump for logging, tagging fill, and other monitoring related purposes.